Improve read-datoms fault tolerance recovery by use of seek-datoms

[lbradstreet]

It may be possible to improve the way read-datoms recovers by seeking directly rather than dropping datoms until we've hit checkpointed index.

See http://docs.datomic.com/clojure/#datomic.api/seek-datoms

It may be a little complex doing this correctly, as it may depend on what index is used e.g. :avet, :vaet, etc.